1. Field
Embodiments of the present invention relate to an electronic disc brake, and more particularly to, a disc brake which may stably return a piston when braking operation is terminated and improve the effect of return of the piston.
2. Description of the Related Art
In general, a disc brake is a device that is used to slow or stop a vehicle by forcibly pressing friction pads against both sides of a disc rotating together with a wheel of the vehicle to stop rotation of the disc. Recently, an electronic parking brake system configured to electronically control operation of a parking brake is installed to a disc brake to perform a parking brake function.
To reduce brake drag that is caused by continuous contact between the disc and the friction pads after the braking operation, two solutions may be considered for such disc brakes. One solution is to move the piston backward using a sealing member and a roll-back chamfer. The other solution is to move the piston backward by applying a compression spring to pad plates, which are pressed against the disc.
FIG. 1 is a cross-sectional view illustrating a conventional electronic disc brake, and FIG. 2 is a view illustrating operation of moving a piston backward using a sealing member and a roll-back chamfer.
As shown in FIGS. 1 and 2, an electronic disc brake 1 includes a pair of pad plates 11 to generate braking force by being pressed against a disc D, a carrier 10 to support the pair of pad plates 11, a caliper housing 20 installed at the carrier 10 and provided with a piston 22 slidably installed in a cylinder 21 to apply pressure to the pair of pad plates 11, a motor 50 to generate driving force, a spindle unit 40 to convert rotational force of the motor 50 into linear movement and apply pressure to the piston 22, and a sealing member 30 interposed between an inner surface of the cylinder 21 and an outer surface of the piston 22. The sealing member 30 serves to seal the space between the inner surface of the cylinder 21 and the outer surface of the piston 22 to prevent leakage of brake oil and to allow the piston 22 to be moved forward in the direction of arrow A, as shown in FIG. 2, for implementation of braking and moved back to an original position thereof by resilience of the sealing member 30, by which the sealing member 30 recovers an original shape thereof, when the braking operation is terminated.
The sealing member 30 is formed in a ring shape to be inserted into a circular roll-back chamfer 23 provided in the inner surface of the cylinder 21, and has a quadrilateral cross section. In addition, the roll-back chamfer 23 to accommodate the sealing member 30 is provided with a slope 24 at a side of the opening facing the direction opposite to the direction of braking (indicated by arrow A) to allow the sealing member 30 to be deformed when the piston 22 is moved forward in the direction of braking, as shown in FIG. 2.
When the piston 22 is moved forward to perform braking, the sealing member 30 is deformed as shown in FIG. 2. When the braking operation is terminated, the sealing member 30 returns in the direction opposite to arrow A to an original state thereof shown in FIG. 1. Accordingly, the piston 22 is moved back to an original position thereof by resilience of the sealing member 30. This action is referred to as roll-back. The return distance of the piston 22 depends on the deformed width L of the sealing member 30.
However, in the conventional electronic disc brake 1, since the side of the sealing member 30 facing the direction in which braking is performed is a flat surface roughly perpendicular to the outer surface of the piston 22 as shown in FIG. 1, the deformed width L of the sealing member 30 is limited as shown in FIG. 2, and thereby the return distance of the piston 22 may be short. That is, the deformation of the conventional sealing member 30 is small, and thus increasing the return distance of the piston 22 may be limited. If the return distance of the piston 22 is short, the friction pads 12 attached to the pad plates 11 may not be sufficiently separated from the disc D when the braking operation is terminated, resulting in wear of the friction pads 12. That is, the amount of roll-back given according to the machining allowance of the cylinder 21 and friction between the sealing member 30 and the piston 22 may result in high drag.
Further, in the case in which the spindle unit 40 applies pressure to the piston 22 to generate parking braking force, since braking force is first generated by hydraulic brake pressure and then parking braking force is applied by the spindle unit 40, slippage between the sealing member 30 and the piston 22 may occur higher than when braking force is generated by general hydraulic brake pressure. Thereby, the return distance of the piston 22 may be short, resulting in high drag.
Meanwhile, in the case of using a compression spring (not shown), the amount of roll-back may increase as the friction pad 12 wears, resulting in poor initial brake feeling. Further, excessive roll-back of the piston 22 may occur due to the compression spring, resulting in a sinking brake pedal.